The invention relates to a method for purifying phosphorus, as a result of which the area of application of products produced from phosphorus, can be expanded due to a great reduction in sulfur content.
Generally, organic impurities up to 0.3% are contained in technical grade white phosphorus. These impurities interfere in some areas of application with further processing and must therefore be removed before subsequent reaction. According to GDR patent 76,482, removal of the interfering organic impurity is possible by a treatment of the phosphorus with sulfuric acid. The phosphorus to be purified is intensively mixed briefly with highly concentrated sulfuric acid in a special apparatus. During this mixing, an emulsion is formed. In order to separate the purified phosphorus rapidly from the emulsion, the latter is passed into warm water. A rapid separation of the phosphorus is required in order to avoid explosive reactions between phosphorus and sulfuric acid. To remove enclosed sulfuric acid, the phosphorus is passed through a packed column. Due to its higher specific gravity, the phosphorus sinks countercurrently to the warm water. With this method, the phosphorus is largely freed from organic impurities.
A relatively large increase in sulfur content of the phosphorus, purified in this manner, is due to the reaction of the sulfuric acid with the phosphorus during the purification method and is disadvantageous for some applications. In general, concentrations of 500 ppm and, in rare cases, concentrations as high as 2,000 ppm are attained. In a side reaction, a small portion of the sulfuric acid is reduced by the phosphorus and therefore leads to an increase in the sulfur content of the phosphorus. According to an older purification method, the phosphorus is also treated with concentrated sulfuric acid. The sulfuric acid, however, is separated off on the basis of its higher specific gravity and without the addition of water. This method also leads to a large increase in the sulfur content of the phosphorus. With this procedure, sulfur contents of more than 500 ppm have been detected. In certain applications, the high sulfur content leads to undesirable side reactions. For example, hydrogen sulfide is formed as a by-product when phosphorus is converted to phosphane. It is also disturbing that such phosphorus, upon contact with water, gives off poisonous hydrogen sulfide and thereby makes handling and transport more difficult. No method for removing sulfur from white phosphorus, which is suitable for industrial application, is described in the literature. Since phosphorus is an extremely reactive chemical, problems were to be expected with the removal of sulfur.